Reference is made to my book entitled "Internal Reflection Spectroscopy" published 1967 by Harrick Scientific Corp., Ossining, N.Y., whose contents are incorporated by reference. The book describes the basic techniques of internal reflection spectroscopy (IRS). Chapter IV describes and illustrates various forms of internal reflection elements (IREs) for use in IRS. A typical IRE comprises a flat plate with square side edges and bevelled ends for receiving a beam of radiation (IR, UV, or visible) which propagates down the length of the IRE via multiple internal reflections from the major plane surfaces, the evanescent wave at the reflection points or areas interacting with a sample located on or in contact with a major surface. The resultant interaction modulates the radiation beam which upon exiting from the IRE can be procesed in the spectrometer and upon detection processed electrically to produce a spectrum of radiation beam intensity as a function of beam wavelength which is characteristic of the sample material. Because the radiation within the IRE is usually not collimated, the beam also travels transversely to the long axis of the plate with the result that some of the non-axial or skew rays will strike the IRE edges surfaces, which are typically parallel, plane and orthogonal to the major surfaces. If the edge surfaces are unpolished, a substantial fraction of the radiation is scattered and lost upon reflection from the edges. This results in reduced beam energy or radiation intensity throughput. It is common to polish the edge surfaces to eliminate light scattering by the edge surfaces. But this has the disadvantage that the edge surfaces then become sensitive to materials in contact with them, meaning that the evanescent wave present upon total internal reflection from the polished edge surfaces interact with any material in contact therewith and therefore also contribute to beam modulation. This produces spurious interfering spectra from the IRE holder, O-ring seals, and adhesives used to bond the IRE in its special holder typically along the IRE edge surfaces.
Attempts have been made to eliminate these spurious spectra by placing reflecting metal foils between the IRE edge surfaces and the holder, O-ring seals, adhesives, etc. In other instances, metal reflecting strips, usually of aluminum, have been deposited along the edge surfaces, including the top and bottom, of the IRE. However, the deposition process is a very tedious and expensive process. Moreover, a metallized surface is not a perfect (100%) reflector, and, therefore, energy loss results due to the presence of the metal. For example, the reflectivity of aluminum in air at a wavelength of 2500 cm is 97.5%, while the reflectivity of a zinc selenide (a typical IRE material)-aluminum interface is only 94%. But, after 10 reflections from the ZnSe-Al interface, the energy loss is about 50%, in contrast with the energy loss at total internal reflection from a ZnSe-Air interface which is zero. Hence, as far as is known to me, none of these attempts have proven completely successful, even though the problem has existed for well over 20 years.